At least 36 people were killed and several more injured after a coach carrying pilgrims plunged off a motorway flyover in southern Italy, rescue services said today.

A coach carrying pilgrims plunged off a motorway flyover in southern Italy, killing at least 38 people in the worst such accident in western Europe in the last decade.

Local prosecutors on Monday launched an investigation into possible manslaughter over Sunday evening's accident near the town of Avellino, on the main highway between Naples and Bari.

Rescuers were battling to extract passengers from the mangled wreckage in a wooded area off the road, where a row of beige seats patterned with blue swirls lay, streaked with blood.

Passengers' belongings, including a hat, shoes, and a child's teddy bear littered the ground.

The coach, carrying 48 people including children, rammed several cars on a busy dual carriageway before it plunged off a viaduct through a crash barrier and down a slope about 50 kilometres (30 miles) from Naples, in an area described as an accident black spot.

President Giorgio Napolitano described the accident as "an unacceptable tragedy" and called for improved road safety standards.

Police said 38 people had died, including the driver, although transport minister Maurizio Lupi had put the number of dead at 39.

Another 10 passengers were injured, along with another nine people in cars hit by the coach before it careered of the road.

The group was returning from a pilgrimage to Pietrelcina in the Campania region of southern Italy, the birthplace of Padre Pio, an Italian priest canonised in 2002 and worshipped in the country's south.

Bodies of many of the dead had been laid out under white sheets, while a small wooden cross was left by a wellwisher, propped up near a bunch of roses by the wreckage.

Relatives and friends comforted each other outside the morgue in the nearby town of Monteforte Irpino as they heard the fate of their loved ones.

The accident was the deadliest in western Europe in a decade and the worst since an October 2010 incident in Ukraine when 45 people died.

Prime Minister Enrico Letta, on a visit to Athens, said it was a "very sad time" for Italy and observed a minute's silence in honour of the victims before addressing a conference in the Greek capital.

"This tragedy has profoundly moved our country... it is an open wound," he said. "I am grieving for and express my profound sorrow to the families of the victims".

Italian news agency ANSA said the manslaughter probe would look into the possible role of the driver, as well as the state of the coach and the crash barrier on the highway.

Ansa said the driver's body would be examined for the possible presence of alcohol or drugs while traffic police have seized the vehicle documents from the coach operator Mondotravel.

Rescue workers said they had pulled 33 bodies from the wreckage and found three more thrown from the vehicle as it plunged 30 metres (100 feet) down a slope.

Another two died in hospital of their injuries.

"Our men are working to save as many lives as possible," fire chief Pellegrino Iandolo said.

"We are still trying to extract people from the vehicle," a police spokesman said. "Our priority now is to free the wounded."

Photographers at the scene said about a dozen wrecked cars littered the highway.

"Looking down from the overpass, the scene of the tragedy: some 30 bodies covered by white sheets, lined up along the roadside," said Cesare Abbate of Italy's ANSA news agency.

One survivor, quoted by his uncle who met him in hospital, reported hearing a tyre exploding and that the driver had been unable to control the vehicle.

The Naples-Bari highway has been closed to traffic, the police said.

The last major coach accident in Europe was in March 2012 in Switzerland, when a coach carrying Belgian schoolchildren home from a skiing holiday crashed, killing 28 people, including 22 children.

The accident also comes just days after a train crash in Spain last Wednesday which killed 79 people, the deadliest rail disaster in the country in decades.

The driver appeared in court on Sunday on 79 counts of reckless homicide over the crash near the pilgrimage city of Santiago de Compostela, northwest Spain.